Recently, in the field of metal processing oil, hydraulic oil and others, widely used is a water-based lubricant oil for the purpose of preventing lubricant oil leakage and heat generation and for preventing fire owing to lubricant oil scattering. In a water-based lubricant oil such as typically a solution-type cutting oil or a water-glycol-based hydraulic oil, used is a polyether compound as a lubricity enhancer and a viscosity improver. The water-based lubricant comprising a polyether compound is advantageous in that its appearance is transparent, it is noncombustible, it is highly stable at a high temperature and it is free from troubles of separation and decay. As the polyether compound for such applications, much used is a random copolymer of ethylene oxide and propylene oxide. There is known another example of using a block polymer of ethylene oxide and propylene oxide as a poorly-foamable polyether compound; and for example, Patent Document 1 discloses a metal processing oil composition containing a so-called reverse-block-type polyether produced by adding propylene oxide to polyethylene glycol. Patent Document 2 discloses a water-soluble cutting oil composition containing a PO-EO-PO structure (where PO means an oxypropylene group, and EO means an oxyethylene group) block-type polyoxyalkylene compound, a fatty acid having from 8 to 10 carbon atoms, and a basic compound.
However, all such polyether compounds heretofore used in the art are not fully satisfactory in point of the lubrication performance and the defoamability thereof.
On the other hand, recently, studies and developments of dendritic polymer compounds have been positively made as the hyperbranch structures of those compounds can promote expression of various functions and wide-range application and development thereof can be expected. The dendritic polymer compounds may be grouped into a dendrimer and a hyperbranch-type polymer.
The hyperbranch-type polymer is a generic term for hyperbranch polymer compounds having a branched structure in the recurring unit thereof. The dendrimer is a monomolecular compound precisely controlled through poly-stage synthetic reaction, while the hyperbranch-type polymer is a synthetic polymer compound generally obtained in a single-stage polymerization process.
Regarding the hyperbranch-type polymer, a hyperbranch-type polymer produced through ring-opening polymerization is known. The polymerization is referred to as multibranching polymerization in which the monomer itself does not have a branch and the branch in the resulting polymer is formed through ring-opening reaction.
As one example of the hyperbranch-type polymer produced through ring-opening polymerization, a hyperbranch-type polyether is reported, which is produced through ring-opening reaction of a cyclic ether for branch formation and molecular chain growth (for example, see Non-Patent Documents 1 and 2). According to this report, a hyperbranch-type polyglycerol is produced through anionic ring-opening polymerization of glycidol using a potassium alkoxide/initiator system. As the initiator, for example, used is 1,1,1-tris(hydroxymethyl)propane, 1-dibenzylamino-2,3-dihydroxypropane, or the like; and a hyperbranch-type polyglycerol having an absolute molecular weight of from 1,000 to 1,000,000 and a degree of molecular weight dispersion of smaller than 1.8 is produced.
However, the method is problematic in that a branched structure is used as the initiator, the method requires complicated operation and its reproducibility is poor, and the physical properties of the polymer comprising pure monomer units are not clear.
Glycidol has one epoxy group and one hydroxyl group, and therefore it may be considered as a latent AB2-type monomer.
It is also reported to produce a hyperbranch-type polyglycerol through cationic ring-opening polymerization of glycidol using a cationic reagent (for example, see Non-Patent Document 3). In this case, however, the produced hyperbranch-type polyglycerol has some problems in that the molecular weight thereof is from 700 to 10,000 or so and is small and the degree of molecular weight dispersion thereof is larger than 2, and these are difficult to control.